Variable reactance electron tube circuit



1954 R. B. WOODBURY VARIABLE REACTANCE ELECTRON TUBE CIRCUIT Filed Sept. 27, 1945 FIG! INVENTOR ROGER B. WOODBURY BY Kw \Y & LL

ATTORNEY Patented Oct. 5, 1954 VARIABLE REACTANCE ELECTRON TUBE .CIRCUIT Roger B. Woodbury; Boston, Mass, assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application September 27, 1945, Serial No. 618,979

Claims.

My-i-nvention relates in general to adjustable impedance elements and more particularly to those voltage controllable impedance elements classified as reactance tubes.

A reactance tube is an application of an electron tube in an alternating current circuit which provides at the input terminals thereof an impedance which may be varied in magnitude and phase angle by the variation of a tube element voltage. Reactance tubes are widely used to control the frequency of oscillators as in automatic frequency control and for the generation of frequency modulated electric waves.

R.eactance tubes heretofore utilized in-alternating current circuits, have had the disadvantage of introducing a dissipative component thereby lowering the selectivity of an associated tuned circuit. A further disadvantage has been the limited impedance variation obtainable for wide variation in control potential.

It is therefore a specific object of my present invention to provide a reactance tube variable impedance having substantially negligible loss.

Another object of my invention is to provide a reactance tube having an impedance substantially uniformly adjustable over a predetermined range by means of a control potential.

A further object of my invention is to provide a reactance tube circuit which through the utilization of a control circuit and control potential transforms a substantially fixed electrical impedance to uniformly variable impedance.

- A still further object of my invention is to providea reactance tube in the form of a high input impedance cathode follower circuit havin an obtainable input capacitance variation from zero to the full value of a fixed capacitor.

These and other objects of my invention will now become apparent from the following detailed specification taken in connection with the accompanying drawings in which:

Fig. 1 illustrates the low loss reactance tube circuit of my invention;

Fig. 2 illustrates a modified reactance tube and control circuit therefor, operative to control the frequency of an oscillator; and

Fig. 3 illustrates a further modification of the reactance tube and control circuit illustrated in Fi 1.

Referring now to Fig. 1, there is illustrated a reactance tube cathode follower ll arranged to provide between terminal l2 and ground; a variable capacitive reactance for controlling the resonant frequency of a tuned circuit comprising inductor l3 and capacitor Id. The cathode follower circuit includes a load resistor 15 of comparatively low resistance between the cathode and ground. The anode of cathode follower I I is directly connected to a positive power source. The reactance tube input terminal 12 is connected to the control grid ofthe cathode follower ll through a comparatively large blocking or coupling capacitor l6; and is also connected to the cathode of the cathode follower throughsmall fixed capacitor H. A reactancecontrolling potential is applied to the control grid of the cathode follower H through the high grid, resistance 2|. This control voltage is preferably adjustable and slightly negative in its center value, however, but need not be so limited as:alternating voltage may be used to obtain a desired frequency effect.

If parallel inductor l3 and capacitor l4 constitute the tuned elements of an oscillator or the like, an alternating potential will exist there-- across and will be impressed upon the control grid of'cathode follower I! in series with the negative control potential to which capacitor 16 is charged. For a well designed cathode follower l I, the gain is substantially equal to unity. Consequently, if the reactance control potential is such that the cathode follower conducts throughout the alternating current cycle, the alternating potential at the control grid and thus at terminal I2 will at all times be equal to the alternating potential at the tube cathode. Under these conditions the potential across capacitor I! is substantially zero, no current flows therethrough, and the reactance tube circuit has no effect upon the natural resonant frequency of tuned'circuits l3, l4.

If the control potential applied to the cathode follower control grid is made negative by an amount required to cutoff cathode follower I l for a fraction of the alternating current cycleythe absence of current flow through load resistor l5 will cause the cathode potential to .fall to zero during the cutoff period. Thus, during the cutoff period the series circuit of capacitor I1 and cathode load resistor 15 are shunted'directly across the tuned circuit between terminal l2 and ground. If sufliciently negative, the control voltage will cutoif the cathode follower ll throughout the cycle and thus continuously shunt capacitor ll across the tuned circuit. Hence, the time period during which capacitor H is rendered effective is determined by the instantaneous-magnitude of the control voltage.

The-effective Q. of the shunting reactance tube circuit is dependent upon the relative values of capacitor l1 and resistor l5 and the particular frequency of operation. Mathematically, this shunting circuit Q is equal to It is thus clear that the use of a small capacitor I1 and comparatively small resistor IE will result in an extremely high Q or low loss impedance for frequency control purposes as viewed from the input terminal [2.

Summarizing, the operation of the reactance tube circuit of Fig. l is such as to render a fixed capacitor effective for a predetermined fraction of an alternating current cycle between the limits of zero time and the full cycle. During that portion of the cycle when capacitor l! is ineffective, the resonant frequency is determined by the tuned circuit alone. lower II is cut off, the resonant frequency is changed by an amount determined by the shunting capacitor 11 in series with resistor IS. The output signal thus may be considered a particular frequency for a fraction of a cycle and a substantially different frequency for the remainder of the cycle. The average output frequency is lower than the tuned circuit natural frequency due to the average effect of the shunting capacitor. The application of this capacitor may be regarded as providing an instantaneous shift frequency and phase of the output signal in the direction of lower frequency. For frequency control over a comparatively narrow range, the output signal distortion resulting from this phase shift is no greater than the average non-linear distortion normally introduced by an electron tube. When the reactance tube is utilized for frequency control over a wide band by using a comparatively large capacitor 11, the output distortion is somewhat increased, but may be compensated for by suitable filtering.

The application of a control voltage directly to the control grid of the cathode follower H through resistor 2| introduces a dissipative network which shunting the resonant circuit has the effect of lowering the Q thereof. In Fig. 2 there is shown a circuit wherein the reactance tube principles described in connection with Fig. 1 are utilized for controlling frequency and in which the control voltage is applied through an extremely high impedance circuit. In Fig. 2 an electron tube 3| is connected as a stabilized Hartley oscillator having a tuned circuit comprising a tapped inductor 32 and capacitor 33,

shunted by the input circuit of a reactance tube 34. As described in connection with Fig. 1, the reactance tube 34 comprises a cathode follower having a cathode load resistor 35 and a small fixed capacitor 36 connected between the cathode and the input terminal 31. The input terminal 31 is connected through large blocking capacitor 4| to the control grid of a second cathode follower 42, having a load resistor 43 and a bias circuit comprising parallel resistor 44 and capacitor 45. A grid leak 46 is connected between the control grid of the cathode follower 42 and the high potential end of the load resistor 43. Since the gain of cathode follower 42 is substantially equal to unity, the voltage output appearing across load resistor 43 is the same as that applied to the control grid and this output signal is coupled to the control grid of reactance tube cathode follower 34 through a coupling capacitor 5|. Frequency, or reactance controlling potential is also applied to the control grid of cathode Whenever the cathode folthereof.

follower 34, through a high resistance 52. As in the example of Fig. l, the potential applied to the control grid of cathode follower reactance tube 34 is equal to the sum of the alternating signal voltage and the control potential. However, in the circuit illustrated in Fig. 2, the terminal 31 at the reactance tube input has been isolated from the shunting impedance of the control voltage source. A variation in negative control potential applied through resistor 52 to the cathode follower 34 controls the portion of the cycle during which capacitor 36 is rendered effective to shunt the tank circuit 32, 33 and thus determine the frequency.

In Fig. 3 there is shown an alternative method of eliminating the dissipation due to the loading of the reactance tube circuit by the grid leak resistor through which control voltage is applied. In this circuit, inductor GI and capacitor 62 form a parallel resonant circuit, the frequency of which is controlled by the reactance tube 63. The reactance tube 63 is a cathode follower as described in connection with Fig. l, and a capacitor 64 is connected between grid and cathode The anode of the cathode follower E3 is returned to a positive voltage source, and the cathode is returned through a load resistor 65 to a negative voltage supply. The application of control potential to the cathode follower 63 is effected by a control tube 66 having its anode and cathode in parallel with the corresponding elements of cathode follower 63. The control grid of tube 66 is connected to the source of control voltage with which it is desired to determine the frequency of the resonant circuit 6|, 62. In operation, if the control voltage applied to control tube 66 is such as to permit conduction therethrough, current flows through resistor 65 and raises the cathode potential of reactance tube 63 to the point of cutoff. Under these conditions, the capacitor 64 shunts the tuned circuit to lower the resonant frequency thereof. As the control potential applied to cathode follower 66 is made negative, the voltage drop in resistor 65 is decreased to the point where conduction takes place in reactance tube 63 for a fraction of the alternating current cycle. During the conduction period the grid and cathode of the reactance tube 63 are substantially at the same potential and the capacitor 64 is rendered ineffective insofar as changing the frequency of the resonant circuit is concerned. It will thus be noted that the potential controlling the conduction of the cathode follower 63 is applied through a high impedance tube circuit 66, thereby mainimizing the effect of the control voltage source impedance in loading the tuned circuit. It will also be noted that the control voltage changes for obtaining a frequency change in a given direction are of opposite phase in Figs. 2 and 3. In Fig. 2 a positive increase in control potential results in a decrease of tuned circuit resonant frequency, whereas in Fig. 3, a corresponding change in control potential results in a decrease of the resonant frequency.

It will be understood that the method of obtaining a variable, controllable capacitance, as described in connection with Figs. 1, 2 and 3 need not be thus limited. As an example, similar circuits may be used for providing controllable inductance, resistance, or combinations thereof. Evidently, various modifications and extensions of the principles hereinabove set forth may become apparent to those skilled in the art. I prefer therefore that the spirit and' scope of the present invention be defined by the appended claims.

I claim:

1. An electron tube circuit for efiectively providing a variable capacitance of substantially negligible loss for an alternating current circuit comprising a first electron discharge tube having at least an anode, a cathode and a control grid and connected as a cathode follower circuit with substantially unity gain, a fixed capacitor connected between control grid and cathode of said first electron discharge tube, a second electron vacuum tube havingat least an anode, a control grid and a cathode, a resistor connected as a common load to the cathodes of said first and second electron discharge tubes, and a bias controlling voltage applied to the control grid of said second electron discharge tube operative to determine the portion of each alternating current cycle during which said fixed capacitor is rendered effective, said second electron discharge tube isolating said first electron discharge tube control grid from said controlling voltage.

2. In an alternating current circuit, a frequency control system for a resonant circuit comprising a fixed capacitor shunting said resonant circuit, a normally conducting electron tube con nected as a cathode follower and having at least an anode, a cathode and a control grid, said capacitor being coupled between said control grid and said cathode so that the same alternating current potential is applied to both terminals of said capacitor during conduction of said tube, an adjustable source of direct current potential, and means applying said direct current potential to the control grid of said electron tube to bias said tube to cut-01f for a predetermined portion of each alternating current cycle, whereby said capacitor shunts said resonant circuit during said cut-off period.

3. In an alternating current circuit, a frequency control system for a resonant circuit comprising, a cathode follower circuit including a normally conducting electron tube having a cathode, a control grid and an anode with a load resistor connected between cathode and ground, means coupling the alternating potential of said resonant circuit to the control grid of said electron tube, a fixed capacitor connected with said load resistor to form a series circuit shunting said resonant circuit, said cathode follower circuit during conduction of said tube maintaining said cathode at a potential substantially equal to the alternating potential coupled to said control grid, thereby applying substantially zero potential across said capacitor, a source of control potential, and means applying a voltage from said source to bias said electron tube to non-conduction for a predetermined period of said alternating current cycle, whereby said capacitor effectively shunts said resonant circuit to change the resonant frequency thereof periodically.

4. In the circuit as claimed in claim 3 wherein said coupling means includes a second cathode follower circuit including a second normally conducting electron tube having a cathode, a control grid and an anode with a second load resistor connected in the circuit between cathode and ground, said control grid of said second electron tube being coupled to said resonant circuit, and said control grid of said first named electron tube being coupled to said second load resistor whereby the impedance of said control potential source is isolated from said resonant circuit.

5. In an alternating current circuit, a frequency control system for a resonator circuit comprising, a cathode follower circuit including a normally conducting electron tube having a cathode, a control grid and an anode with a load resistor connected between cathode and ground, means coupling the alternating potential of said resonant circuit to the control grid of said electron tube, a fixed capacitor connected with said load resistor to form a series circuit shunting said resonant circuit, said cathode follower circuit during conduction of said tube maintaining said cathode at a potential substantially equal to the alternating potential coupled to said control grid, thereby applying substantially zero potential across said capacitor, a source of control potential, a second electron tube having a cathode, control grid and an anode, the anode of said second tube being connected directly to said anode of said first named tube and the cathode of said second tube being connected to said cathode of said first named tube, and means applying a voltage from said source to bias said second electron tube to conduction a predetermined amplitude of space current, whereby the space current of said second tube flowing in said common load resistor biases said first named electron tube to nonconduction for a predetermined period of said alternating current cycle, thereby effectively shunting said resonant circuit periodically with said capacitor to change the resonant frequency thereof.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,950,759 Terman Mar. 13, 1934 2,182,377 Guanella Dec. 5, 1939 2,274,648 Bach Mar. 3, 1942 2,323,598 Hathaway July 6, 1943 

